iMX6 SDMA under passed WML

I am using SDMA with Serial. I have configured channel descriptor to use the script app_2_mcu to receive data from Serial RX FIFO.

I am successful whenever the data is equivalent to Water Mark Level but when the data is less than WML, I do get interrupt with the help of bit ATDMAEN in serial. Also, data is copied to destination but for some reason I am not able to figure how much data is read by DMA. I have tried to read the buffer descriptor count field in hope that it will be updated by script with the updated value, but it is showing me the original value that I programmed.

Is there a way to know how much data is copied by DMA in case of we are under the WML and Aging timer bit of serial is triggered?